The present invention relates to high areal recording density magnetic recording media exhibiting enhanced thermal stability and increased signal-to-medium noise ratio (xe2x80x9cSMNRxe2x80x9d). The invention finds particular utility in the form of hard disks such as employed in high areal recording density magnetic data/information storage and retrieval devices and systems.
Magnetic recording (xe2x80x9cMRxe2x80x9d) media and devices incorporating same are widely employed in various applications, particularly in the computer industry for data/information storage and retrieval applications, typically in disk form. Conventional magnetic thin-film media, wherein a fine-grained polycrystalline magnetic alloy layer serves as the active recording medium layer, are generally classified as xe2x80x9clongitudinalxe2x80x9d or xe2x80x9cperpendicularxe2x80x9d, depending upon the orientation of the magnetic domains of the grains-of magnetic material.
A conventional longitudinal recording, hard disk-type magnetic recording medium 1 commonly employed in computer-related applications is schematically illustrated in FIG. 1, and comprises a substantially rigid, non-magnetic metal substrate 10, typically of aluminum (Al) or an aluminun-based alloy, such as an aluminum-magnesium (Alxe2x80x94Mg) alloy, having sequentially deposited or otherwise formed on a surface 10A thereof a plating layer 11, such as of amorphous nickel-phosphorus (Nixe2x80x94P); a seed layer 12A of an amorphous or fine-grained material, e.g., a nickel-aluminum (Nixe2x80x94Al) or chromium-titanium (Crxe2x80x94Ti) alloy; a polycrystalline underlayer 12B, typically of Cr or a Cr-based alloy, a magnetic recording layer 13, e.g., of a cobalt (Co)-based alloy with one or more of platinum (Pt), Cr, boron (B), etc.; a protective overcoat layer 14, typically containing carbon (C), e.g., diamond-like carbon (xe2x80x9cDLCxe2x80x9d); and a lubricant topcoat layer 15, e.g., of a perfluoropolyether. Each of layers 10-14 may be deposited by suitable physical vapor deposition (xe2x80x9cPVDxe2x80x9d) techniques, such as sputtering, and layer 15 is typically deposited by dipping or spraying.
In operation of medium 1, the magnetic layer 13 is locally magnetized by a write transducer, or write xe2x80x9cheadxe2x80x9d, to record and thereby store data/information therein. The write transducer or head creates a highly concentrated magnetic field which alternates direction based on the bits of information to be stored. When the local magnetic field produced by the write transducer is greater than the coercivity of the material of the recording medium layer 13, the grains of the polycrystalline material at that location are magnetized. The grains retain their magnetization after the magnetic field applied thereto by the write transducer is removed. The direction of the magnetization matches the direction of the applied magnetic. field. The magnetization of the recording medium layer 13 can subsequently produce an electrical response in a read transducer, or read xe2x80x9cheadxe2x80x9d, allowing the stored information to be read.
Efforts are continually being made with the aim of increasing the areal recording density, i.e., the bit density, or bits/unit area, and signal-to-medium noise ratio (xe2x80x9cSMNRxe2x80x9d) of the magnetic media. For example, the SMNR may be increased by reducing the grain size of the recording media, as by utilization of appropriately selected seed and underlayer structures and materials, and by reduction of the thickness of the magnetic recording layer. However, severe difficulties are encountered when the bit density of longitudinal media is increased above about 20-50 Gb/in2 in order to form ultra-high recording density media, such as thermal instability, when the necessary reduction in grain size exceeds the superparamagnetic limit. Such thermal instability can, inter alia, reduce and cause undesirable decay of the output signal of hard disk drives, and in extreme instances, result in total data loss and collapse of the magnetic bits.
One proposed solution to the problem of thermal instability arising from the very small grain sizes associated with ultra-high recording density magnetic recording media, including that presented by the superparamagnetic limit, is to increase the crystalline anisotropy, thus the squareness of the magnetic bits, in order to compensate for the smaller grain sizes. However, this approach is limited by the field provided by the writing head.
Another proposed solution to the problem of thermal instability of very fine-grained magnetic recording media is to provide stabilization via coupling of the ferromagnetic recording layer with another ferromagnetic layer or an anti-ferromagnetic layer. In this regard, it has been recently proposed (E. N. Abarra et al., IEEE Conference on Magnetics, Toronto, April 2000) to provide a stabilized magnetic recording medium comprised of at least a pair of ferromagnetic layers which are anti-ferromagnetically-coupled (xe2x80x9cAFCxe2x80x9d) by means of an interposed thin, non-magnetic spacer layer. The coupling is presumed to increase the effective volume of each of the magnetic grains, thereby increasing their stability; the coupling strength between the ferromagnetic layer pairs being a key parameter in determining the increase in stability.
However, a significant drawback associated with the above approach is the discontinuous character of each of the AFC-coupled ferromagnetic layers of the media Specifically, if the magnetic grains of the upper and lower magnetic layers are not grown in vertical alignment, or if they are not of equal size, the areas written in each of the pair of ferromagnetic layers may not coincide. In addition, the prior art approaches to media design fail to adequately take into account the significant effect on stability of magnetic recording media arising from interactions between magnetic grains.
Accordingly, there exists a need for improved methodology and structures for providing thermally stable, high areal recording density magnetic recording media, e.g., in the form of hard disks, with increased signal-to-media noise ratios (SMNRs), e.g., longitudinal media which methodology and media structures can be implemented/fabricated at a manufacturing cost compatible with that of conventional manufacturing technologies for forming high areal recording density magnetic recording media.
The present invention, therefore, addresses and solves problems attendant upon forming high areal recording density magnetic recording media, e.g., in the form of hard disks, which media utilize magnetic or anti-ferromagnetic coupling between spaced-apart pairs of ferromagnetic layers for enhancing thermal stability and increasing SMNR, while providing full compatibility with all aspects of conventional automated manufacturing technology. Moreover, manufacture and implementation of the present invention can be obtained at a cost comparable to that of existing technology.
An advantage of the present invention is an improved, high areal recording density magnetic recording medium having enhanced thermal stability.
Another advantage of the present invention is an improved, high areal recording density magnetic recording medium exhibiting an increase signal-to-medium noise ratio (xe2x80x9cSMNRxe2x80x9d).
Yet another advantage of the present invention is an improved, high areal recording density magnetic recording medium having enhanced thermal stability and SMNR arising from magnetic or anti-ferromagnetic coupling between spaced-apart continuous and discontinuous ferromagnetic layers.
Additional advantages and other features of the present invention will be set forth in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from the practice of the present invention. The advantages of the present invention may be realized as particularly pointed out in the appended claims.
According to one aspect of the present invention, the foregoing and other advantages are obtained in part by a magnetic recording medium having increased thermal stability and signal-to-medium noise ratio (SMNR), comprising a layer stack including, in sequence:
(a) a continuous ferromagnetic layer;
(b) a non-magnetic spacer layer; and
(c) a discontinuous ferromagnetic layer;
wherein the continuous ferromagnetic layer (a) and the discontinuous ferromagnetic layer (c) are coupled together across the non-magnetic spacer layer.
According to embodiments of the present invention, the continuous ferromagnetic layer (a) comprises a material with a very low amount, e.g.,  less than 3-5 at. %, of non-magnetic phases, to ensure strong magnetic coupling between adjacent grains, and wherein, if the continuous layer has magnetic domains which are much larger than the average grain size in the discontinuous layer, the magnetocrystalline anisotropy is greater than about 107 erg/cm3 for reducing the width of the magnetic domain walls thereof to less than or similar to the width of the grains of the discontinuous layer, i.e.,  less than 100 xc3x85, or, if the continuous layer is comprised of strongly coupled single domain grains, the magnetocrystalline anisotropy thereof is greater than about 106 erg/cm3; the continuous ferromagnetic layer (a) having a lower coercivity than that of the discontinuous magnetic layer (c), being from about 10 to about 200 xc3x85 thick and comprising an alloy material selected from the group consisting of Co3Pt, MnAl, Nd2Fe14B, SmCo5, Sm2Co17, Sm2Fe17(N,C), Co100-x-y-zCrxPtyBz, Co100-x-y-z-wCrxPtyNbzTaw, and (Pt,Pd)(Co,Fe)L10 phase; and the non-magnetic spacer layer (b) is up to about 30 xc3x85 thick.
In accordance with embodiments of the present invention, the non-magnetic spacer layer (b) is from about 2 to about 30 xc3x85 thick and, depending upon its thickness, provides ferromagnetic or anti-ferromagnetic coupling (xe2x80x9cAFCxe2x80x9d) between the continuous ferromagnetic layer (a) and the discontinuous ferromagnetic layer and comprises a material selected from the group consisting of ruthenium (Ru), rhodium (Rh), iridium (Ir), chromium (Cr), copper (Cu), and their alloys.
According to embodiments of the present invention, the discontinuous ferromagnetic layer (c) is from about 10 to about 300 xc3x85 thick, also has a large magnetocrystalline anisotropy, i.e.,  greater than 106 erg/cm2, for obtaining sufficient coercivity at lower saturation magnetization, includes exchange de-coupled or partially coupled magnetic grains, and comprises an alloy material selected from the group consisting of CoCr; CoCr with one or more added elements selected from Pt, Ta, B, Mo, Ru, Si, Ge, and Nb; Fe; and Ni.
In accordance with embodiments of the present invention, the magnetic recording medium further comprises:
(d) a substrate having at least one surface for supporting the layer stack; and
(e) non-magnetic seed and underlayers intermediate the at least one surface of the substrate (d) and the layer stack for controlling the crystallographic texture of at least one of said ferromagnetic layers of the layer stack;
wherein the substrate (d) comprises a non-magnetic material selected from the group consisting of Al, Al-based alloys, NiP-plated Al, other non-magnetic metals, other non-magnetic metal alloys, glass, ceramics, glass-ceramics, polymers, and laminates and composites thereof; and the non-magnetic seed and underlayers (e) comprise materials selected from the group consisting of Nixe2x80x94Al, Nixe2x80x94Alxe2x80x94Ru, Nixe2x80x94Alxe2x80x94Ti, Fexe2x80x94Al, Ruxe2x80x94Al, CoTi, Ta, Taxe2x80x94N, Cr, Crxe2x80x94Ta, Crxe2x80x94W, Crxe2x80x94Mo, Crxe2x80x94V, Crxe2x80x94Ti, Crxe2x80x94Ru, and Crxe2x80x94Ruxe2x80x94Ta.
According to a particular embodiment of the present invention, the discontinuous ferromagnetic layer (c) of the layer stack is proximate the at least one surface of the substrate (d); and the medium may further comprise a stacked layer pair intermediate the discontinuous ferromagnetic layer (c) of the layer stack and the non-magnetic seed and underlayers (e), the stacked layer pair consisting of a continuous ferromagnetic layer (a) or a discontinuous ferromagnetic layer (c) adjacent the non-magnetic seed and underlayers (e) and a non-magnetic spacer layer (b) adjacent the discontinuous ferromagnetic layer (c) of the layer stack.
In accordance with another particular embodiment of the present invention, the continuous ferromagnetic layer (a) of the layer stack is proximate the at least one surface of the substrate (d); and the medium may further comprise a stacked layer pair intermediate the continuous ferromagnetic layer (a) of the layer stack and the non-magnetic seed and underlayers (e), the stacked layer pair consisting of a continuous ferromagnetic layer (a) or a discontinuous ferromagnetic layer (c) adjacent the non-magnetic seed and underlayers (e) and a non-magnetic spacer layer (b) adjacent the continuous ferromagnetic layer (a) of the layer stack.
Embodiments of the magnetic media provided by the present invention may further comprise:
(f) a protective overcoat layer over the layer stack; and
(g) a lubricant topcoat over the protective overcoat layer.
According to another aspect of the present invention, a magnetic recording medium exhibiting enhanced thermal stability and increased signal-to-medium noise ratio (SMNR) comprises:
(a) a layer stack including, in sequence:
(i) a continuous ferromagnetic layer;
(ii) a non-magnetic spacer layer; and a
(iii) a discontinuous ferromagnetic layer;
wherein the continuous ferromagnetic layer (i) and the discontinuous ferromagnetic layer (iii) each comprises a material having a very high magnetocrystalline anisotropy greater than about 106 erg/cm3 for obtaining sufficient coercivity at lower saturation magnetization and minimizing the width of the magnetic domain walls, respectively, the continuous ferromagnetic layer (i) has a lower coercivity than that of the discontinuous ferromagnetic layer (iii), the non-magnetic spacer layer provides magnetic or anti-ferromagnetic coupling between the continuous ferromagnetic layer (i) and the discontinuous ferromagnetic layer (iii) depending upon its thickness, and the discontinuous ferromagnetic layer includes exchange de-coupled or partially coupled magnetic grains;
(b) a substrate having at least one surface for supporting the layer stack; and
(c) non-magnetic seed and underlayers intermediate the at least one surface of the substrate (b) and the layer stack (a) for controlling the crystallographic texture of at least one of the ferromagnetic layers of the layer stack.
According to embodiments of the present invention, either the continuous ferromagnetic layer (i) or the discontinuous ferromagnetic layer (iii) of the layer stack (a) is proximate the at least one surface of the substrate (b), and the medium may further comprise a stacked layer pair intermediate the layer stack (a) and the non-magnetic seed and underlayers (c), the stacked layer pair consisting of a continuous ferromagnetic layer (i) or a discontinuous ferromagnetic layer (iii) adjacent the non-magnetic seed and underlayers (c) and a non-magnetic spacer layer (ii) adjacent the layer stack (a).
In accordance with further embodiments of the present invention, the magnetic recording medium further comprises:
(d) a protective overcoat layer over the layer stack (a); and
(e) a lubricant topcoat over the protective overcoat layer.
Still another aspect of the present invention is an improved magnetic recording medium, comprising:
(a) a layer stack comprising at least a pair of spaced-apart continuous and discontinuous ferromagnetic layers; and
(b) means for enhancing the thermal stability and signal-to-medium noise ratio (SMNR) of the medium.
Additional advantages and aspects of the present invention will become readily apparent to those skilled in the art from the following detailed description, wherein embodiments of the present invention are shown and described, simply by way of illustration of the best mode contemplated for practicing the present invention. As will be described, the present invention is capable of other and different embodiments, and its several details are susceptible of modification in various obvious respects, all without departing from the spirit of the present invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as limitative.